1. Field of the Invention
The present invention relates to wireless communication radio frequency (RF) circuits for transmitting and receiving radio waves and communication apparatuses including the same.
2. Description of the Related Art
FIG. 7 is a block diagram showing an example of the configuration of a wireless communication RF circuit included in a communication apparatus, along with a control circuit (baseband circuit). A wireless communication RF circuit 30 includes a receiving antenna 31, a transmitting antenna 32, a demodulator circuit 33, and a modulator circuit 34. The wireless communication RF circuit 30 is connected to a control circuit 35.
The demodulator circuit 33 detects a radio wave received by the receiving antenna 31, extracts a signal wave, and outputs the extracted signal wave to the control circuit 35. The modulator circuit 34 mixes a transmission signal (signal wave) supplied from the control circuit 35 with a carrier and outputs the resulting carrier to the transmitting antenna 32. The transmitting antenna 32 wirelessly transmits the modulated wave.
FIG. 8 shows a specific example of the configuration of a wireless communication RF circuit having the above-described circuit configuration. The RF circuit is described in Japanese Unexamined Patent Application Publication No. 8-116289. Referring to FIG. 8, numeral 101 denotes a receiving antenna, numeral 102 denotes a transmitting antenna, numeral 103a denotes a detector, numeral 104 denotes a modulator, and numeral 108a denotes a control circuit (CPU).
In the configuration shown in FIG. 7, the receiving antenna 31 and the transmitting antenna 32 are separately provided. The receiving and transmitting antennas 31 and 32 are larger, in size than the demodulator circuit 33 and the modulator circuit 34. In addition, the antennas 31 and 32 must be arranged at a distance of approximately λ/2 (λ represents the wavelength of the transmitted/received radio wave) from each other in order to prevent interference therebetween. Specifically, for example, when the frequency of the transmitted/received radio wave is in the 2.4 GHz band, the antennas 31 and 32 must be separated by approximately 6 cm from each other. When the frequency of the transmitted/received radio wave is in the 5 GHz band, the antennas 31 and 32 must be separated by approximately 3 cm from each other. Since the antennas 31 and 32 must be separated from each other, the wireless communication RF circuit 30 is difficult to miniaturize.
As shown in FIG. 9, a transmitting-and-receiving antenna 36 is provided, and the demodulator circuit 33 and the modulator circuit 34 are connected to the antenna 36 via a transmission/reception selector switch 37. In this case, the number of antennas is reduced, which enables miniaturization of the wireless communication RF circuit 30. On the other hand, an SPDT (Single Pole Double (Dual) Throw) switching circuit serving as the switch 37 is provided. Upon signal reception, the SPDT switching circuit switches to the demodulator circuit, and upon signal transmission, the SPDT switching circuit switches to the modulator circuit. Since the configuration of the SPDT switching circuit is complex, the circuit configuration is complicated. Insertion loss in the switch 37, which produces an increase in loss, is substantial. Specifically, if the modulator circuit 34 is a reflective modulator circuit, upon signal transmission, a signal passes through the SPDT switching circuit a total of two times, that is, when the signal is supplied from the antenna 36 to the modulator circuit 34 and when the signal is reflected by the modulator circuit 34 towards the antenna 36. In this case, since the signal passes through the SPDT switching circuit twice, loss is generated, and loss caused by the SPDT switching circuit is greatly increased.
FIG. 10 shows a specific example of an RF circuit with the configuration of the RF circuit shown in FIG. 9. The RF circuit is described in Japanese Unexamined Patent Application Publication No. 9-298484.